Force limiting device and method

ABSTRACT

The present invention relates to a method and apparatus for limiting the contact force between a moving device and another object, using a parallel mechanism and torque limiters where the threshold force to activate the force limiting mechanism is not related to the configuration of the moving device or the location of the contact force relative to the activation point of the force limiting mechanism, and where the mechanism may be configured for one, two or three degrees of freedom. A counterbalance mechanism is also provided to counteract gravity load when the force limiting mechanism is configured for three degrees of freedom and responsive to contact forces including a vertical element. In particular, the invention relates to a method and apparatus for limiting the contact force between a moving robotic device and a contactable object.

TECHNICAL FIELD

The present invention relates to a method and apparatus for limiting theforce between a moving device and another object, using a parallelmechanism and torque limiters where the threshold force is not relatedto the device configuration, and in particular to limiting the collisionor impact force between a moving robotic device and a contactableobject.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. patentapplication Ser. No. 12/627,407 filed Nov. 30, 2009, which is herebyincorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The use of industrial robots is well established in applications wherethe robots are in controlled environments where they are separated byfences or cages. A collision between a robot and a contacted object is acomplex situation, where one of the most severe situations is when acollision with clamping occurs, entrapping the contacted object betweenthe contacting portion of the robot and a static fixture, such as awall. In this case, the severity of the collision may be indexed usingthe maximum contact force, which can then be used as a reference forunexpected collisions during the design process.

Control and dependability are characteristics needed from a robot toallow it to interact in a minimally controlled, e.g., unfenced anduncaged, environment with other objects, which may be moving orstationary. Roboticists may typically use three different strategies todevelop these characteristics. First, roboticists may develop algorithmsthat use vision systems, proximity sensors or the like to anticipate andavoid potentially harmful contacts between robots and objects. Secondly,methods may be developed to detect a collision by monitoring jointtorques or a robot skin and to quickly react to manage the contactforces under a certain level. Thirdly, roboticists pursue robot designsthat will intrinsically prevent damaging contact.

Avoidance, reaction and design strategies can be combined together toimprove robot design. However, the first two options alone may not fullyguarantee the desired result. Consider that a robot intended to interactphysically with a contactable object will require the ability todistinguish desirable and undesirable contacts, e.g., good and badcontacts. This can be done either by disabling sensors on the robotparts intended to interact or by running an algorithm that will decideif the upcoming contacts are desirable or not. In either case, controlis compromised either by unprotecting certain parts of the manipulatoror by giving the robot some sort of “judgment capability” which may insome situations be wrong. Furthermore, avoidance and reaction strategiesrely on electronic components that can fail. Finally, one could arguethat an operator may feel insecure working with a machine protected onlyby an algorithm. Thus, a third strategy may be employed to obtaincompliant and dependable robots, which is to use a design strategy,e.g., to design robots that intrinsically prevent damaging contact.

To design robots to intrinsically prevent damaging contact, a typicalapproach is to make the robot compliant, to reduce the peak contactforce attained during a collision. Compliance may also extend theduration of the contact, allowing the controller to sense a collisionand react to reduce potential damages, within certain constraints, i.e.,reaction time. However, adding compliance may limit the precision andstiffness of the robot, compromising performance and precision.

Some robots designed to avoid damaging contact incorporate a flexibleflange with breakaway function that links the tool to the manipulator.This device triggers an emergency stop when the contact force at thetool control point exceeds a certain threshold, which may be a breakawaytorque measured at the flange. This device therefore limits the moment,not the force that can be transmitted by the manipulator to theend-effector, which means that the threshold depends on the location ofthe collision point. Therefore, for a breakaway system, the design ofthe system must be sub-optimized for the worst case moment arm, whichmay result in a system which is overly sensitive and prone to falsetriggering in high inertia non-collision situations, which may requirelimitations on robot velocity.

Active compliance systems are in some aspects derived from admittancecontrol techniques, e.g., efforts are measured at the effector andprocessed to command a displacement equal to the contact force dividedby a virtual spring stiffness. Thus, the robot behaves like a springaround its trajectory. However, the response time of traditionalactuators is larger than what is required to accommodate high frequencyforces applied during collisions. Consequently, during a collision, therobot may not achieve a compliant behavior and thus this technique isnot optimal as a design strategy.

Techniques may be used to provide passive compliance, at each joint ofthe robot, which may be programmable or non-linear. Programmable passivecompliance consists of using a compliant joint for each axis of therobot and a supplemental set of actuators to allow the adjustment of thestiffness of each joint. Either two antagonistic actuators or a secondactuator that adjusts the stiffness via a mechanism may be used, toallow high stiffness and precision at low velocity and low stiffness athigh velocity, i.e. when contact with the manipulator may be moresevere. This gives the controller the ability to continuously adjust thecompromise between control and performance. However, using this type ofpassive compliance system adds weight and complexity to the manipulator.Also, for many mechanisms, the ratio between the largest stiffness andlowest stiffness is not sufficient to obtain high precision at lowvelocity, when collisions are less severe and high precision is requiredfor acceptable robot performance.

SUMMARY OF THE INVENTION

Nonlinear passive compliance uses a method which places on each joint amechanism whose compliance varies by purely mechanical means. By placinga mechanical device, such as a torque limiter, in series with each jointactuator, the resulting manipulator will be rigid unless external forcesapplied on it exceed a certain threshold, in which case the joint willbecome compliant. This technique allows the design of robots that arestiff and accurate in normal operating conditions, but compliant whencollisions occur. Moreover, this principle is realized mechanically,which means that the reliability of this system does not depend onelectronic components. However, this method is not optimized. By addinga torque limiter on each joint of a serial robot, the force thresholdwill depend on the configuration of the manipulator, because therelation between external forces and articular torques is determined bythe Jacobian matrix of the manipulator, which is generally a function ofthe manipulator's pose. The threshold will also depend on the contactlocation and on the force orientation, which is not optimal since itmeans that the compliance level will vary throughout the robot'sexternal surface.

Therefore, a force limiting device that comprises torque limiters placedin a Cartesian architecture provides numerous advantages over itsarticular counterpart. Provided herein is a force limiting device, whichin a preferred embodiment improves the compliance level of suspendedrobots with constant end-effector orientation relative to the gravitydirection, such as robots performing the Schonflies motions, as relatedto physical object-robot interfaces. The force limiting device includesa parallel mechanism with torque limiters which provide a rigidconnection between the robot and its end-effector during normaloperation, e.g., during non-collision conditions. When the robotend-effector contacts, as in a collision, a resisting object, thetransmitted contact force activates the torque limiters of the forcelimiting device to yield a compliant connection between the robot andits end-effector, reducing the contact force and the severity of theimpact. The force limiting device presented herein may be configured forone, two and three degrees of freedom (DOF) (respectively, 1-DOF, 2-DOF,3-DOF).

In a preferred embodiment, the device provided herein is configured foruse with an overhead robot to limit the collision force resulting fromcontact with the manipulator, end-effector and/or parts other than therobot's end-effector, such as tooling and/or payload, suspended from theoverhead robot. Suspended robots of the type discussed herein are oftenfound in manufacturing plants, and may also be found in otherapplications, such as hospital and medical applications and warehousingapplications, for example.

As provided herein, if an excessive force in a direction correspondingto the degrees of freedom (DOFs) of the force limiting device is appliedduring a collision, the force limiting mechanism is activated and thusthe end-effector is free to move relative to the robot and typicallyopposite to the direction of contact or collision. The activation of themechanism is detected and brakes are applied to stop further motion ofthe robot in the direction of contact. The inertia of the parts locatedkinematically upstream of the force limiting device, e.g., the inertiaof the robot from which the force limiting device is attached and theend effector is suspended, is thus removed from the collision. Also, fora quasi-static collision in which an object is clamped between the robotand a wall or other static fixture, the maximum contact force is theactivation force of the force limiting device for that orientation, asdetermined by the configuration of the torque limiters in the mechanism.Hence, the contact force is reduced to improve damage control for alltypes of blunt collisions.

Architectures are provided herein for force limiting devices configuredfor one, two and three degrees of freedom (respectively, 1-DOF, 2-DOF,3-DOF). The 1-DOF force limiting device provided herein includes asingle parallelogram linkage that could be used when the robot's motionin one direction is more prone to contact than in other directions. The2-DOF force limiting device provided herein includes four legs that formtwo parallelograms and thus behaves similarly to the 1-DOF mechanism.The 1-DOF and 2-DOF force limiting devices, as configured, are notsensitive to the weight of the suspended manipulator or end-effector andthus do not require gravity force compensation. A 2-DOF force limitingdevice may be especially appropriate for applications in industryrequiring large and fast horizontal motion and small and slow verticaldisplacements.

The 3-DOF force limiting device presented herein is configured based ona Delta architecture. The 3-DOF force limiting device can be appliedmore generally than the 1-DOF and 2-DOF mechanisms because it may reactto collisions occurring in any direction on the end-effector, e.g., itmay react to contact forces independent of orientation of the contactforce to the end-effector. Methods to compensate the effect of gravityon the 3-DOF delta configuration, where required when the end-effectorweight or payload weight combined with the end-effector weight is largecompared to the maximum static force limit that is imposed, are providedherein. Other possible configurations for a 3-DOF force limiting deviceare also provided within the scope of the claimed invention.

The force limiting mechanisms provided herein have a force thresholdthat is independent from the contact point on the end-effector, incontrast to known “moment limiting” devices. The force limiting devicesalso allow a larger displacement of the end-effector, which increasesthe distance and time available to mechanically stop a heavy overheadmounted manipulator or robot located above the force limiting device andsuspended end-effector after a threshold contact force is detected.Because the nonlinear Cartesian compliance mechanism of the forcelimiting device described herein will react to Cartesian efforts, thepolytope of the achievable forces will not be dependent on the pose ofthe contacting or colliding mechanism, e.g., the end-effector, and theforce limiting device may be optimized by appropriately selecting amechanism architecture (1-DOF, 2-DOF, 3-DOF) and by appropriatelyselecting limit torques for the torque limiters incorporated therein. Tooptimize the effectiveness of the force limiting device, the mechanismshould preferably be isotropic, such that the achievable forces polytopewill be a square in 2D or a cube in 3D.

The Cartesian mechanism (force limiting device) provided herein may beconstructed using a parallelogram mechanism architecture incorporatingtorque limiters. A parallelogram architecture has the advantage of beingstiffer than serial mechanisms. In a preferred embodiment, the forcelimiting Cartesian mechanism is placed between the robot and itsend-effector. Thus, contact force is reduced to reduce damage andimprove compliance for collisions between a contactable object and anyportion of the end-effector or manipulator that is located upstream fromthe force limiting mechanism in the robot or manipulator's kinematicchain, e.g., between the contacted object and the force limiting device.For a robot or manipulator suspended on an overhead rail-bridge system,this configuration provides comprehensive protection against collisionsof the manipulator and its payload with a contactable object, which maybe an operator, by operatively disconnecting the end effector orcolliding portion from the robot or overhead manipulator upstream fromthe force limiting device, so as to release the rigid connection betweenthe end effector and overhead robot from which the end effector issuspended through the force limiting device.

Further, because the 1-DOF and 2-DOF mechanisms presented herein are notaffected by gravity forces, the force limiting device may be effectivewithout limiting the payload to be carried by the robot. This is alsothe case for the 3-DOF architectures when a gravity compensatingmechanism is included, as provided herein. However, accelerations of therobot may induce inertial forces that may activate the torque limitersof the force limiting mechanism. Thus, for a given load, accelerationsmust be limited to a certain level to prevent the force limiting devicefrom activating such that the end-effector becomes compliant, e.g.,constructively disconnected, during movement in the absence of acollision. The maximum velocity that can be typically imposed on a robotis the maximum velocity that corresponds to blunt, unconstrainedcollisions which may be qualified as compliant. This “compliant”velocity is usually very low for heavy robots. However, if during acollision the end-effector is disconnected from the robot, e.g., therigid connection between the robot and end effector is released so as tobecome compliant, the effective inertia to which the contacted object issubjected is then greatly reduced. Therefore, it can be assumed thatusing a force limiting mechanism as provided herein will allow anincrease in the maximum velocity of a robot moving in an environmentwith potential for contactable object-robot interaction or physicalobject-robot interaction. This maximum velocity should typically beevaluated using a collision model that considers a broad spectrum ofcollision parameters, including, typically, the way the robot reactswhen a collision is detected (braking force, delay before the brakes areapplied, etc.).

As noted previously, collisions in which a contactable object is clampedto a wall or against another fixed object by a robot can be most severe.The force limiting mechanism described herein effectively reduces themaximum clamping force that the robot can apply in quasi-staticcondition to a force level determined by the limit torque levels set aslimits for the torque limiters incorporated in the force limitingdevice. As the velocity of the robot system increases beyond aquasi-static, or very low velocity condition, compliance is improvedbecause the inertia impacting the contactable object against the wall ina clamping condition is reduced. Because the force limiting mechanism isunable to store elastic potential energy, the robot will not continue topush on the contacted object after the collision has taken place and theforce limiting device has been activated. This is an advantage since itwill help the movement of the robot away from the contacted body afterthe collision.

As discussed previously, some robots incorporate a flexible flange withbreakaway function to limit the moment, not the force, transmitted bythe manipulator to the end-effector during a collision. These systemsare often sub optimized for the worst case moment arm, resulting in alimited velocity, overly sensitive system prone to false triggering ofthe breakaway mechanism and deteriorated performance. In contrast, thecollision force required to activate the Cartesian force limiting deviceprovided herein is constant across the entire end-effector collisionspace, e.g., the activation force does not vary with a moment arm, asdescribed further herein. This activation behavior is preferable since acollision generally occurring anywhere on the end-effector will bereacted to at a relatively constant activation force, optimizing therobot design by allowing the minimum activation force to be maximizedwhile reducing the sensitivity of the force limiting mechanism tonon-collision inertia during robot movement. As an additional advantage,the force limiting mechanism provided herein has a large achievabledisplacement compared to the breakaway device, which yields the space,and therefore reaction time, required by a heavy overhead mountedmanipulator to stop prior to non-compliant contact with the objectinvolved in the collision.

A force limiting device configured to limit the contact force of amoving object with a contacted object is provided herein. The forcelimiting device includes a first attachment, which may be an upperplatform or interface, a second attachment, which may be a lowerplatform or interface and one or more parallelogram linkages. The forcelimiting device may be connected at a first end to the first attachmentand may be connected at a second end to the second attachment, using oneor more connection points. The connection points may include rotatablejoints. The connection of the parallelogram linkage to the attachmentsmay be through a rotatable joint or through an intermediate segment,such as a leg connected to the parallelogram linkage at one end and tothe attachment at the other end. The one or more parallelogram linkagesestablish the orientation of the first attachment to the secondattachment; e.g., the first attachment may be oriented to be parallel tothe second attachment with the same orientation relative to their commonnormal axis.

The force limiting device may include two parallelogram linkages whichare arranged to be perpendicular to each other, such that the axes ofthe planes of the parallelogram linkages are coincident. Theparallelogram linkages are configured to transmit an input torque, wherethe input torque is, for example, a couple resultant from a forceagainst the suspended portion attached to the second attachment opposingthe movement of the robot attached to the first attachment.

The force limiting device may be attached to the moving portion and thesuspended portion of a robot, to operatively connect the moving portionand the suspended portion of the moving object. The force limitingdevice is configured to maintain a rigid orientation between the firstattachment and the second attachment when the input torque is less thanthe activation torque; and to be activated when the input torquetransmitted through one or more of the parallelogram linkages exceeds anactivation torque. When the force limiting device becomes activated, itis configured to become compliant and thereby cause the contact forcebetween the moving object and the contacted object to be decreased. Theforce limiting device may be compliant by compliance of theparallelogram linkage.

Alternatively, the joints of each of the one or more parallelogramlinkages may be rotatable. One of the rotatable joints of eachparallelogram linkage may include a torque limiting mechanism configuredto activate the force limiting device when the input torque exceeds atorque limit. Alternatively, a torque limiter may be substituted for oneof the rotatable joints of each parallelogram linkage, or may beoperatively included at an attachment point between the force limitingdevice and the robot. The torque limit of the torque limiting mechanismis typically set equivalent to the activation torque.

The force limiting device may include three or more parallelogramlinkages arranged in a Delta configuration, e.g., a parallel armarrangement. In a preferred embodiment, the parallelogram linkages areconfigured so as to be spaced 120 degrees equidistant from each other;however spacing with angles different than 120 degrees is understood tobe within the scope of the claimed invention. The Delta configured forcelimiting device may further include a gravity compensating mechanismconfigured to include a spring, an actuator, a counterbalance systemwhich may include counterweights and pulleys, or a combination of these.The gravity compensating mechanism may be configured to compensate forthe weight of the suspended portion, the weight of a payload, which maybe variable, or the combined weight of the suspended portion and apayload.

The force limiting device may be included in a robot system adapted foroverhead suspension, the system including a robot capable of moving, anda suspended portion which is operatively connected to and suspended fromthe robot. The force limiting device may be attached between the robotand the suspended portion, to operatively connect the robot and thesuspended portion. When the suspended portion, which is manipulated andmoved by the robot, exerts a contact force against an object, anactivation force is inputted to the suspended portion opposing thecontact force. The force limiting device is configured to becomeactivated and when activated, become compliant when the activation forceexceeds an activation level. When the force limiting device becomescompliant, the contact force exerted by the suspended portion againstthe contacted object is immediately and substantially decreased, toprevent or minimize damage to the contacted object. In a workspaceincluding robots, the contacted object may be another piece of equipmentor stationary fixture.

The force limiting device provided herein increases the compliance levelof physical object-robot interactions. As would be understood by thoseskilled in the art to be within the scope of the claimed invention, theforce limiting device may be used in other environments, for example,controlled (fenced or gated) work cells, to minimize damage to the robotsystem, robot, end-effector, manipulator, payload, tooling, otherobjects and equipment in the workspace, etc. by minimizing the force ofnon-intended moving robot-to-moving object or moving robot-to-stationaryobject collisions. The force limiting device may be used in any scenariowhere a release of a rigid connection in response to an activation forceis desirable to alleviate or minimize damage resulting from a collisionwith one or move moving or static objects incorporating the forcelimiting device.

The above features and advantages and other features and advantages ofthe present invention are readily apparent from the following detaileddescription of the best modes for carrying out the invention when takenin connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic perspective view of a robot system including aforce limiting device, showing the robot system in motion toward anobject adjacent a stationary object such as a wall;

FIG. 1B is a schematic perspective view of the robot system of FIG. 1Awith the robot system end effector contacting an object in a clampingcollision;

FIG. 1C is a schematic perspective view of the robot system of FIG. 1Awith the force limiting device activated;

FIG. 2A is a schematic perspective view of a force limiting device in a1-DOF parallelogram configuration;

FIG. 2B is a schematic perspective view of the force limiting device ofFIG. 2A in an activated condition;

FIG. 3 is a schematic perspective view of a force limiting device in a2-DOF parallelogram configuration;

FIG. 4A is a schematic perspective view of a force limiting device in a3-DOF delta configuration;

FIG. 4B is a schematic perspective view of the force limiting device ofFIG. 4A configured for gravity compensation;

FIG. 5 is a schematic plan view of a gravity compensating device for usewith the force limiting device of FIG. 4A; and

FIG. 6 is a schematic plan view of a counterbalancing system for usewith the force limiting device of FIG. 4A.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Provided herein is a force limiting device to increase the compliancelevel of suspended robots, as related to physical object-robotinterfaces. The force limiting device described herein is aparallelogram mechanism with torque limiters which provide, duringnon-collision operation, a rigid connection between the robot and itsend-effector. If an excessive force in a direction corresponding to thedegrees of freedom (DOFs) for which the force limiting device isconfigured is applied during a collision, the force limiting mechanismis activated and the end-effector becomes compliant, e.g., is free tomove relative to the robot and typically opposite to the direction ofthe collision. Brakes or like functioning devices are applied to stopfurther motion of the robot in the direction of contact when the forcelimiting device is triggered or activated. The inertia of the movingrobot located kinematically upstream of the force limiting device isthus removed from the collision, and for a quasi-static collision, themaximum contact force is the activation force of the force limitingdevice for that orientation, as determined by the configuration of thetorque limiters in the mechanism.

In a preferred embodiment, the device provided herein is configured foruse with an overhead mounted robot to limit the collision forceresulting from contact with the end-effector, robot arm, tooling and/orpayload suspended from the robot. Different architectures are providedfor force limiting devices configured for one, two and three degrees offreedom (respectively, 1-DOF, 2-DOF, 3-DOF).

Referring to the drawings, wherein like reference numbers refer to likecomponents, shown in FIGS. 1A, 1B and 1C is a schematic perspective viewof a robot system 100 including a force limiting device 110. The robotsystem 100 includes a robot 105, which is suspended from an overheadsupport system, which may be a ceiling or an overhead rail system. Robotsystem 100 further includes a force limiting device 110 operativelyconnected to robot 105 through an interface 150, and a suspended portion115, which may be a robot arm, end-effector or similar mechanism. Thesuspended portion or end-effector 115, which is operatively connected tothe lower portion of force limiting device 110, may include additionaltooling and/or include a payload. Robot system 100 may also include, aswould be understood by those skilled in the art, a control system, whichmay include a controller, controls, sensors and other mechanismscommonly included in a robot system. Robot system 100 further includes abraking mechanism 113 to stop motion of robot 105. The braking mechanism113 of robot 105 may be initiated by the robot controls or by triggeringthe force limiting device 110.

Referring to FIG. 1A, shown is the robot system 100 moving in adirection 125 toward an object 130. In the arrangement shown, object 130is positioned between a stationary object, such as a wall 117, and therobot system 100 such that continued movement of robot system 100 indirection 125 will result in contact of the robot arm or end-effector115 with the object 130. As described previously, end-effector 115 issuspended from force limiting device 110, which is operatively attachedthrough interface 150 to robot 105. Force limiting device 110 includes ajointed parallelogram linkage 111 in which one revolute joint isreplaced with a torque limiter 120, which will be described further.Under the conditions shown in FIG. 1A, there is no torque input intotorque limiter 120 with respect to robot 105 and end-effector 115,therefore torque limiter 120 maintains the parallelogram linkage 111 offorce limiting mechanism 110 in a static or rigid state, thusmaintaining a rigid connection between end-effector 115 and robot 105.

As shown in FIG. 1B, robot 105 has continued in direction 125 such thatend effector 115 has made contact with object 130 at contact point 135,in a clamping collision, whereby object 130 has become clamped againstthe stationary fixture or wall 117. As robot 105 continues movement indirection 125, end effector 115 exerts an increasing contact force onobject 130 at contact point 135. Object 130 exerts an opposing force atcontact point 135 on end effector 115 which is transmitted to forcelimiting device 110, including the joint replaced by torque limiter 120.When the opposing force exerted against end effector 115 at the point ofcontact 135 in combination with the continued movement of robot 105 indirection 125 results in a torque input to torque limiter 120 exceedingits torque limit or trigger point, the force limiting device 110 isactivated.

Upon activation of the force limiting device 110, and referring now toFIG. 1C, two events occur. First, the torque limiter 120 releases,resulting in the compliant movement of the parallelogram linkage 111 offorce limiting device 110 in a direction 140 which immediately andsubstantially decreases and/or relieves the contact force exerted by endeffector 115 on object 130. Secondly, the activation of force limitingdevice 110 triggers the braking mechanism 113 of robot 105 to stop anymovement of robot 105 in direction 125. Thus, by activating forcelimiting device 110, damage to object 130 from contact with end effector115 may be minimized or avoided.

The nonlinear Cartesian compliance mechanism 110, or force limitingdevice 110 can therefore be said to operatively “disconnect” theend-effector 115 from the robot 105 over a certain distance when acollision occurs, that distance being determined by the geometry offorce limiting device 110 as it complies, releasing and/or relieving thecontact force of end-effector 115 from the contacted object, which maybe a object 130. The operative “disconnection” of end-effector 115 overa certain distance or space provides the distance and time required tostop the movement of the robot 105 and robot system 100, thussubstantially or fully relieving the collision force at contact point135. As shown in FIGS. 1A-1C, the force limiting device 110 as providedherein is well suited for applications such as robots suspended onrail-bridge systems, however may be applicable to other systems andconfigurations within the scope of the claimed invention.

Returning to FIGS. 1A-1C, the contact force over time can be describedin phases. The contact force or collision force between the end effector115 and contacted object 130, is zero in FIG. 1A, during thepre-collision phase. As robot system 105 continues movement in direction125, and when end-effector 115 makes contact with object 130, which isentrapped between end-effector 115 and the stationary wall 117 in aninitial collision phase, the contact force at contact point 135increases until the activation torque of torque limiter 120 is attainedand force limiting device 110 is triggered. In a third phase, forcelimiting device 110 becomes compliant, resulting in an immediate andsubstantial decrease in the contact force at contact point 135, due tothe activation of the torque limiter 120. The force required to overcomethe continued movement of robot system 100 is still present, but iseliminated in a final phase when the movement of robot 105 is stopped bytriggering brake 113 concurrently with activation of force limitingdevice 110 during the initial collision phase. So long as robot 105 canbe stopped within the geometric compliance limits of force limitingdevice 110, the inertia from robot 105 does not contribute further tothe contact force on object 130 at contact point 135. The trigger valueor torque limit established and set for torque limiter 120 shouldconsider the geometric compliance limits of the force limiting device110 and the braking dynamics of robot 105, which may be differentdepending on the orientation of the collision angle of the end effector115 with respect to the parallelogram configuration of torque limitingdevice 110.

Referring now to FIGS. 2A and 2B, shown is a schematic perspective viewof a force limiting device 110 in a 1-DOF parallelogram configuration.FIG. 2A shows force limiting device 110 in a non-activated state, wheretorque limiter 120 acts to maintain the parallelogram linkage 111 ofdevice 110 in a rigid configuration. FIG. 2B shows force limiting device110 in an activated state, where the threshold torque has been met torelease torque limiter 120 such that the parallelogram linkage 111reacts compliantly to force 140, the contact force at contact point 135during a collision of the robot system 100 with a fixed or constrainedobject, e.g., the object 130 in the clamping collision illustrated inFIGS. 1A-1C. The force limiting device 110 includes an upper platform orinterface as a first attachable portion 150 which is operativelyattachable to robot 105. FIGS. 2A and 2B provide holes 155 as anattachment interface through which device 110 may be bolted, pinned orriveted, for example, to robot 105. As would be understood, any suitablemeans known to those skilled in the art may be used to fixedly attachthe upper interface 150 of force limiting device 110 to robot 105. Theforce limiting device 110 includes a lower platform or interface as asecond attachable portion 160 which is operatively attachable to robotarm or end-effector 115. FIGS. 2A and 2B provide holes 165 as anattachment interface through which device 110 may be bolted, pinned orriveted, for example, to robot arm or end-effector 115, however anysuitable means known to those skilled in the art may be used to fixedlyattach the lower interface 160 of force limiting device 110 to robot armor end-effector 115.

Referring now to FIG. 2A, the 1-DOF force limiting device 110 shownincludes a single parallelogram linkage 111 that may optimally be usedwhen the motion of robot system 100 in one direction is much moreundesirable than in other directions. FIG. 2A shows a simple 1-DOFnonlinear Cartesian compliance mechanism 110, also referred to as a1-DOF force limiting device 110 mounted between a suspended robot 105and its end-effector 115. Mechanism 110 includes a parallelogram linkage111 in which one revolute joint 185 is replaced with a torque limiter120. The parallelogram linkage consists of two legs 170 attached throughrevolute joints 185, 195 and shafts 180, 190 to platforms 150, 160, andgenerally configured as illustrated by FIG. 2A. Referring to FIG. 2A,the three joints 195 are passive revolute joints which are rotatableabout shafts 190 where shafts 190 are rotatively or fixedly attached toplatforms 150, 160. Alternatively, the joints 195 may be fixedlyattached to shafts 190, where shafts 190 are rotatively attached toplatforms 150, 160. The fourth joint 185 is fixedly attached to inputshaft 180, such that as the parallelogram linkage 111 is subject to agenerally horizontal force (as oriented in FIG. 1A), such as a force 140(see FIG. 2B), the input shaft 180 provides an input torque, such asinput torque 175, to torque limiter 120.

Under normal conditions, the torque limiter 120 restrains the rotationof shaft 180 and thus prevents the parallelogram linkage of forcelimiting device 110 from moving, maintaining a rigid connection betweenrobot 105 and end-effector 115. However, if a contacting collisionoccurs, for example, a collision of the type illustrated in FIG. 1B, thecouple 175 passing through torque limiter 120 exceeds set limits andforce limiting mechanism 110 is activated and moves responsively toforce 140, as shown in FIG. 2B. This compliant movement functionally“disconnects” end-effector 115 from robot 105 with respect to agenerally horizontal plane and thus immediately and substantiallyrelieves the contact force of end effector 115 from the object involvedin the collision, e.g., the object 130 shown in FIGS. 1A-1C. Followingactivation and responsive compliant movement of force limiting device110, the object 130 is only subjected to the inertia of end-effector115, which can be significantly lower than the inertia of the entirerobot system 100. For the force limiting mechanism 110 to be effectivein improving compliance by reducing the contact force on the object, thecollision must be detected and robot 105 must be stopped or brakedbefore the parallelogram linkage 111 of mechanism 110 reaches the end ofits travel, e.g., its geometric limit. The collision can be detectedwith a limit switch (not shown) placed on the mechanism 110, e.g., incontact with one of the parallelogram links and a signal can be sent tothe controller of robot 105 to brake the system, or alternatively, anemergency stop signal can be sent directly to the brake system 113 ofrobot 105 without passing through the robot's controller, thus improvingthe reliability of the system by reducing the risks of electroniccomponent failure. Once robot 105 is stopped, the gravity force of thesuspended robot arm 115 tends to naturally return force limitingmechanism 110 to its original position. One important advantage of theparallelogram architecture of force limiting device 110 is that thecouple passing through the torque limiter 120 only depends on themagnitude of the horizontal force 140 (referring to FIG. 2B) applied onend-effector 115 and is not affected by the height of the point ofapplication of the force, e.g., the distance between contact point 135and torque limiter 120 (see FIG. 1B). This implies that the same forcelevel will cause the activation of the force limiting mechanism 110whether the collision occurs at the top, middle or bottom portion of theobject 130, or at an end or middle portion of end-effector 115. Thisprovides a significant advantage over breakaway systems which aresensitive to length of the moment arm of the contact force, e.g., thedistance of the collision contact point 135 from the actuation point ofthe force limiting mechanism.

FIG. 3 is a schematic perspective view of a force limiting device 210 ina 2-DOF parallelogram configuration. 2-DOF force limiting device 210 hasfour legs 270 that form two parallelogram linkages 211 and thus behavessimilarly to 1-DOF force limiting device 110. 1-DOF force limitingdevice 110 and 2-DOF force limiting device 210, as configured andprovided herein, are not sensitive to the gravity force of the suspendedweight of end-effector 115, 215 and thus do not require a gravitycompensating mechanism. A 2-DOF force limiting device 210 is especiallyappropriate for applications requiring large and fast horizontal motionof a robot system 100 and small and slow vertical displacements.

Referring to FIG. 3, 2-DOF force limiting device 210 has aparallelepipedic architecture making it potentially suitable as amechanism to reduce the collision force for collisions occurring acrossthe horizontal plane, e.g., the X-Y plane of FIG. 3. This isaccomplished using a parallel architecture as generally shown in FIG. 3,which is composed of four identical legs 270 which each include apivoting joint 297 and a pivoting joint 295, where each pair of pivots295, 297 has their axes in two perpendicular, horizontal directions. Thelegs 270 are placed in such a way that the axes of the first set ofpivots 297 intersect at a single point, e.g., the X-Y origin generallyat the center of upper platform 250, two of them sharing the same axis,perpendicular to the axis of the other two. The axes of the second setof pivots 295 are oriented similarly to the first set of pivots 297, ina plane parallel to the X-Y plane defined by the axes of the second setof pivots 297. The parallelogram linkages 211 are operatively attachedto the attachable portion or lower platform 260 through legs 290.Additionally, brackets 222 may be included to operatively attachparallelogram linkages 211 and/or torque limiters 220 to the attachableportion or upper platform 250. Upper platform 250 is fixedly attachableto robot 105 and lower platform 260 is fixedly attachable toend-effector 215. Similar to the attachment configuration discussed fordevice 110, and as shown for the embodiment of device 210 in FIG. 3,upper platform 250 is provided with holes 255 as an attachment interfacethrough which device 210 may be bolted, pinned or riveted, for example,to robot 105, or any suitable means known to those skilled in the artmay be used to fixedly attach the upper interface 250 of force limitingdevice 210 to robot 105. The force limiting device 210 includes a lowerplatform or interface 260 which is operatively attachable to robot armor end-effector 215. FIG. 3 provides holes 265 as an attachmentinterface through which device 210 may be bolted, pinned or riveted, forexample, to robot arm or end-effector 215, however any suitable meansknown to those skilled in the art may be used to fixedly attach thelower interface 260 of force limiting device 210 to robot arm orend-effector 215.

The workspace of force limiting mechanism 210 is a sphere centered onupper platform 250 and the orientation of lower platform 260 remains thesame relative to upper platform 250. Two of the first sets of pivots 297are connected to input attachments 285, to provide input to torquelimiters 220, providing similar behavior and function as 1-DOF mechanism110 described earlier. For the 2-DOF architecture of device 210, onlythree of the four legs 270 are required to kinematically constrainmechanism 210 in a non-collision situation. The fourth leg 270over-constrains mechanism 210, providing the advantage of addingstiffness and reducing the effect of backlash if, for example, thelength of one of the leg 270 is adjusted to provide an internal pre-loadto mechanism 210. Force limiting device 210 presents similar advantagesas force limiting device 110, e.g., the magnitude of contact orcollision force that will activate force limiting mechanism 210 is onlydependent on the orientation and not the height of contact point 135 orthe distance between contact point 135 and torque limiters 220 (see FIG.1B), and after a collision triggering force limiting device 210, gravitytends to return mechanism 210 to its original configuration.

Referring to FIG. 3, the activation sequence of force limiting device210 is similar to that described for force limiting device 110 in FIGS.1A through 2B. Under normal conditions, torque limiters 220 restrain therotation of pivots 295, 297 and shafts 270 and thus prevents theparallelogram linkages 211 of force limiting device 210 from moving,maintaining a rigid connection between robot 105 attached to attachmentplate 250 and end-effector 215 attached to attachment plate 260.However, if a contacting collision occurs, for example, a collision ofthe type illustrated in FIG. 1B, torque is transmitted though theparallelogram linkages of force limiting device 210 to torque limiters220. When this transmitted torque exceeds a set limit for either of thetorque limiters 220, force limiting mechanism 210 is activated and movesresponsively in opposition to the contact force in direction 140 (seeFIG. 1C). When at least one of the torque limiters 220 releases itsrespective input attachment 285 when the input torque from thetransmitted contact force exceed the set limit of the torque limiter220, the force limiting device is released from its rigid configurationand becomes compliant. Legs 270 rotate and pivot using pivots 295, 297responsive to and opposing the contact force. This compliant movementfunctionally “disconnects” end-effector 215 from robot 105 with respectto a generally horizontal plane and thus immediately and substantiallyrelieves the contact force of end effector 215 from the object involvedin the collision, e.g., object 130 shown in FIGS. 1A-1C. Followingactivation and responsive compliant movement of force limiting device210, the object 130 is only subjected to the inertia of end-effector215, which can be significantly lower than the inertia of the entirerobot system 100. For the force limiting mechanism 210 to be effectivein improving compliance by reducing the contact force on the object, thecollision must be detected and robot 105 must be stopped before theparallelogram linkage 211 of mechanism 210 reaches the end of itstravel, e.g., its geometric limit. The collision can be detected with alimit switch (not shown) placed on the mechanism 210, e.g., in contactwith one of the parallelogram links 211 and a signal can be sent to thecontroller of robot 105 to brake the system, or alternatively, anemergency stop signal can be sent directly to the brake system 113 ofrobot 105 without passing through the robot's controller, thus improvingthe reliability of the system by reducing the risks of electroniccomponent failure. Once robot 105 is stopped, the gravity force of thesuspended robot arm 215 tends to naturally return force limitingmechanism 210 to its original position.

Referring now to FIGS. 4A and 4B, shown in FIG. 4A is a schematicperspective view of a force limiting device 310 in a 3-DOF deltaconfiguration; and FIG. 4B provides a schematic perspective view of theforce limiting device 310 of FIG. 4A configured for gravitycompensation. Referring to FIG. 4A, the 3-DOF force limiting device 310presented herein is configured based on a Delta architecture. A 3-DOFforce limiting device 310 can be applied more generally than a 1-DOFforce limiting device 110 or a 2-DOF force limiting device 210 because a3-DOF device 310 can react to collisions occurring in any direction withend-effector 315 including collisions with a vertical force component.Methods to compensate the effect of gravity on a 3-DOF deltaconfiguration, where required when the weight of the end-effector orcombined weight of the payload with the end-effector is large comparedto the maximum static force limit that is imposed, are provided herein.Other possible configurations for 3-DOF force limiting devices are alsoprovided within the scope of the claimed invention.

As discussed previously for force limiting devices 110, 210, forcelimiting device 310 includes an upper platform 350 which is configuredto be fixedly attachable to robot 105 and a lower platform 360 isfixedly attachable to end-effector 315. Similar to the attachmentconfiguration discussed for devices 110, 220 and as shown for theembodiment of device 310 in FIG. 4A, upper platform 350 is provided withholes 355 as an attachment interface through which device 310 may bebolted, pinned or riveted, for example, to robot 105. As is understood,any suitable means known to those skilled in the art may be used tofixedly attach the upper interface 350 of force limiting device 310 torobot 105. The force limiting device 310 includes a lower platform orinterface 360 which is operatively attachable to robot arm orend-effector 315. FIG. 4A provides holes 365 as an attachment interfacethrough which device 310 may be bolted, pinned or riveted, for example,to robot arm or end-effector 315, however any suitable means known tothose skilled in the art may be used to fixedly attach the lowerinterface 360 of force limiting device 310 to robot arm or end-effector315.

Referring to FIG. 4A, shown is a preferred embodiment of a 3-DOF forcelimiting device 310 with a Delta architecture comprising three legs 370,each operatively connected to a torque limiter 320 that positions theupper link 395 of a parallelogram linkage 311 whose lower link 395 isoperatively attached to lower platform 360. Each of the four cornerjoints of each parallelogram linkage 311 are schematically representedin FIG. 4A by a spherical joint, although other types of joints, e.g.,universal joints, may be used in the parallelogram linkage within theclaimed scope of the invention. The parallelogram linkages 311 constrainthe orientation of lower platform 360 in a way such that upper platform350 and lower platform 360 maintain a constant orientation relative toeach other. Force limiting device 310, when activated by a force abovethe preset threshold of any of the torque limiters 320, can performtranslation in the X, Y and Z directions (see FIG. 4A). For theconfiguration of device 310 shown in FIG. 4A, it is assumed that theoptimal design of a force limiting device 310 using the Deltaarchitecture will comprise identical legs 370 that are equally spaced,i.e., placed 120° relative to one another with equal radii forattachment points on the platforms 350, 360. However, it is understoodthat the Delta architecture is not restricted to three legs. Forexample, four or more legs could be used with the same general behavior,although the mechanism would be over constrained. It is furtherunderstood that the legs may be spaced with angles different than 120degrees and yield the same general behavior.

Two kinematic properties of the 3-DOF configuration should be optimizedfor efficiency of force limiting device 310. The first property is theworkspace of force limiting device 310. Since robot 105 must be capableof braking prior to travel a distance exceeding the geometric motionlimit of force limiting device 310, the optimal workspace for device 310will be a sphere centered at its reference point. The radius of thatsphere needs to be equal to the maximum braking distance of robot 105considering collisions occurring in any direction. Secondly, anisotropic Jacobian matrix will give the maximal ratio of the minimumover the maximum forces needed to activate force limiting device 310.The isotropy of the achievable force space is more difficult to obtainfor a 3-DOF mechanism 310, however, it is obtainable for the referencepoint of the mechanism 310 by choosing design parameters assuming theoptimal achievable force polyhedron is a cube.

Referring to FIG. 4A, the activation sequence of force limiting device310 is similar to that described for force limiting devices 110 and 210.Under normal conditions, torque limiters 320 restrain the rotation ofpivots 395 and legs or shafts 370 and thus prevents the parallelogramlinkages 311 of force limiting device 310 from moving, maintaining arigid connection between robot 105 attached to attachment plate 350 andend-effector 315 attached to attachment plate 360. However, if acontacting collision occurs, for example, a collision of the typeillustrated in FIG. 1B, torque is transmitted though the parallelogramlinkages 311 of force limiting device 310 to torque limiters 320. Whenthis transmitted torque exceeds a set limit of any one of the torquelimiters 320, force limiting mechanism 310 is activated and movesresponsively in opposition to the contact force in direction 140 (seeFIG. 1C). When at least one of the torque limiters 320 releases itsrespective pivot point 385 when the input torque from the transmittedcontact force exceed the set limit of the torque limiter 320, the forcelimiting device 310 is released from its rigid configuration and becomescompliant. Legs 370 become rotatable and pivot about pivots 385, 395,and parallelogram linkages 311 move responsively to and opposing thecontact force. This compliant movement functionally “disconnects”end-effector 315 from robot 105 with respect to a generally horizontalplane and thus immediately and substantially relieves the contact forceof end effector 315 from the object involved in the collision, e.g., theobject 130 shown in FIGS. 1A-1C. Following activation and responsivecompliant movement of force limiting device 310, the object 130 is onlysubjected to the inertia of end-effector 315, which can be significantlylower than the inertia of the entire robot system 100. For the forcelimiting mechanism 310 to be effective in improving compliance byreducing the contact force on the object, the collision must be detectedand robot 105 must be stopped before the rotating legs 370 andparallelogram linkages 311 of mechanism 310 reach the end of travel,e.g., the geometric limits of the collective linkage and structure offorce limiting device 310. The collision can be detected with a limitswitch (not shown) placed on the mechanism 310, e.g., in contact withone of the parallelogram links 311 and a signal can be sent to thecontroller of robot 105 to brake the system, or alternatively, anemergency stop signal can be sent directly to the brake system 113 ofrobot 105 without passing through the robot's controller, thus improvingthe reliability of the system by reducing the risks of electroniccomponent failure. Once robot 105 is stopped, the gravity force of thesuspended robot arm 315 tends to return force limiting mechanism 310 toits original position.

The 3-DOF force limiting device 310 as configured in FIG. 4A, which issensitive to collisions from all directions, including collisiondirections with a vertical component, may also be sensitive to thegravity force representing the weight of end-effector 315 and itspayload. The gravity force of end-effector 315 or the combined gravityforce of end-effector 315 with a payload may create a load on torquelimiters 320 that will eventually limit the force that the end-effector315 can apply to accomplish a certain task. Also, the gravity force fromthe weight of the payload and/or end-effector 315 might exceed theactivation limits of force limiting device 310, potentially renderingdevice 310 ineffective in a collision situation. The potential effect ofthe gravity force from the weight of the end-effector 315 can becounteracted using gravity balancing when the combined weight of thepayload and the end-effector 315 is large relative to the activatingcontact force limit for the force limiting device 310, to maintain theeffectiveness of force limiting device 310 in a collision event.

Referring to FIG. 4B, counterbalancing a parallel mechanism such asforce limiting device 310 is usually complex because of the nonlinearand coupled relation between Cartesian and articular displacements. Inthe present situation, however, the force limiting device need only bebalanced for one configuration or condition of use, e.g., suspending theend-effector arm 315 during normal operation. In a collision situation,when the force limited device 310 is activated to respond to the contactforce of the collision from any direction, it is unnecessary tocounterbalance the gravity load of end-effector 315, because theprioritized response in a collision situation is relief of the contactforce, not robot performance. Therefore, it is acceptable to incorporatea counterbalancing mechanism that balances the gravity load from theweight of the end-effector 315 in only the neutral configuration, e.g.,when the arm 315 is suspended and in use during normal non-collisionrobot operating conditions. In this case, and when the gravity forcefrom the weight of arm 315 and any payload is relatively constant, thecounterbalancing mechanism may be simply a pre-loaded spring 398, asshown in FIG. 4B, which provides advantages of mechanical simplicity andlow weight. Notably, this counterbalancing method is valid only if thespring 398 does not limit the workspace of device 310, which might onlybe possible for smaller leg length ratios.

Alternatively, and referring now to FIG. 5, when the gravity force fromthe weight of arm 315 and any payload may be variable, balancing thevariable gravity load with a spring 415 may require an actuator 405 tomodify the position of one of the spring's anchor points 410, where theactuator 405 must provide a force equal to the gravity force. Shown inFIG. 5 and generally indicated at 400, a counterbalance system may beprovided where an actuator 405 could adjust the counterbalance when thegravity force changes, for example, when the robot arm 315 is picking upa new payload. To do so, the upper anchor point 410 of counterbalancespring 415 could be mounted on a linear guide 420 with a locking system410 (included in upper anchor point 410). In normal (unloaded) mode, theanchor point 410 is locked and thus the force limiting device 310 isdisplacing the gravity force of the end-effector 315. However, whenrobot arm 315 picks up or releases a payload, the anchor point 410 ofthe spring is unlocked and the actuator 405 counterbalances the changein gravity force via spring 415, while the force limiting device 310counterbalances the weight of end-effector 315. If, for example, thegravity force increases, the heavier load will pull on spring 415 andanchor point 410 will be adjusted by actuator 405 until the elasticforce in spring 415 reaches a force equal to load's weight. Then, anchorpoint 410 is locked again and the robot system 100 goes back to normaloperating mode where end-effector 315 and its payload can be displacedvertically in the event of a collision with sufficient contact force toactivate force limiting device 310.

Referring now to FIG. 6, a counterbalancing system generally indicatedat 450 is provided, which uses a passive system of counterweights and/orsprings to accomplish the required counterbalancing. The system 450 maybe configured using remote counterweights 470, 475, where the balancerand the counterweights are placed away from robot 105, to avoid addinghorizontal inertia, and the counterbalancing force is transmitted withcables 460, 465 via routing pulleys 490, 485, respectively, designed ina way that horizontal displacements of robot 105 do not movecounterweights 470, 475. This allows the balancing of load 455, whichmay, for example, be comprised of robot arm 315 and a payload, withoutadding inertia for displacements in axes other than the vertical one. Toconfigure the counterbalancing system 450 in this manner, the load thatneeds to be balanced must be separated into two parts, a first load, anda second load 455, by the force limiting device 310. The first load,which is configured to be relatively constant, may include the loadrepresented by robot 105 and device 310. The first load may additionallyinclude the load of an end-effector or robot arm 315 without a payload,in which event pulley 460 would be operatively connected to end-effector315, also. The second load 455 may include the load of a payload, whichmay be variable, and may additionally include the load of end-effectoror robot arm 315, if this load is not included in the first load. Theeffectiveness of the force limiting device 310 in an activatedcondition, e.g., responsive as a force limiting device in the event of acollision, requires that the first and second loads be allowed to moverelative to one another when an activating contact force threshold ismet. This implies that the second load, which is also the portion of theload which may be variable, must be moved with the same actuator as thefirst load under normal (non-collision) conditions. It should be notedthat in the case where the second load is small compared to the level offorce required to accomplish the task, for example, where there is noincremental payload, balancing of the second load is not required andthe system can be counterbalanced with a single pulley system, shown inFIG. 6 as including cable 460 and pulleys 490, counterweight 470 and anactuator 480. However, in the general case, a second balancing or cableand pulley system, as shown in FIG. 6 including cable 465 and pulleys485, will be required to counterbalance the second part of the load 455.This second balancing system adds minimal additional parts andcomplexity to the overall counterbalance system and robot system 100.The first counterweight 470, used to balance the first load, which isthe constant load of the system, is not required to be variable since italways balances substantially the same load, and only one actuator 480is needed. Therefore, balancing a first and second load separatelyrepresents only a limited increase in system complexity.

The particular balancing method employed depends on the particularapplication and anticipated variability of the loads, and whether therobot system 100 uses a balancing system with counterweights for otherpurposes, where, for example, the additional complexity of a pulley andcounterweight counterbalancing system is minimal and limited toincorporating a second routing-pulley system. As discussed previously,using counterbalancing springs provides a simpler mechanical design thatis likely less expensive than using remote counterweights. This isespecially true when the load is constant, since for that situationthere is no need to add a mechanism or actuator to adjust the balancingforce or to change the spring after a certain number of cycles to avoidfatigue. Therefore, determining the most suitable balancing system isdependent on numerous factors that need to be evaluated for eachapplication.

A 3-DOF force limiting device based on Delta architecture, such asmechanism 310 shown in FIGS. 4A and 4B, may be preferably suited for theexemplar scenario provided herein, e.g., as a force limiting deviceoperatively connecting a suspended robot arm to an overhead movingrobot, as shown in FIGS. 1A to 1C, to relieve contact force between therobot arm or end-effector 115 and a substantially stationary object 130in a clamping collision, for a number of reasons. First, the workspaceof a force limiting device configured with Delta architecture such asmechanism 310 is the intersection of three toruses and can be optimizedto center a large spherical volume on the neutral position, maximizingthe available motion in all directions to allow robot 105 to be stoppedbefore reaching the geometric limits of mechanism 310. Second, a forcelimiting device 310 incorporating Delta architecture can be designedsuch that the mechanism will be isotropic in its neutral configuration,optimizing the available force that the robot 100 can apply in anydirection to accomplish a task while limiting the overall maximum staticforce to a certain level, which may be a compliant level, to minimizeimpact on performance. Third, a force limiting device 310 incorporatingDelta architecture is sensitive only to forces, not moments, whichallows the response of the force limiting device to be independent fromthe location of the collision contact point on the end-effector.Further, the device configuration based on Delta architecture isgeometrically compact and simple, making it potentially less costly andmore reliable while limiting its footprint. Notwithstanding theadvantages of a Delta based 3-DOF configuration for the force limitingdevice 310 provided herein, it is understood that other configurationsof 3-DOF force limiting devices with may also be provided within thescope of the claimed invention.

While the best modes for carrying out the invention have been describedin detail, those familiar with the art to which this invention relateswill recognize various alternative designs and embodiments forpracticing the invention within the scope of the appended claims.

1. A method for limiting a contact force of a robot with an object, themethod comprising: operatively attaching a force limiting device betweena robot and a suspended portion; wherein the force limiting device:includes at least one parallelogram linkage including a joint; and isactivated by an activation force; transmitting a contact force to theforce limiting device when the suspended portion contacts an object;activating the force limiting device when the contact force exceeds theactivation force, such that the force limiting device becomes compliant;and decreasing the contact force inputted to the object by the suspendedportion when the force limiting device becomes compliant.
 2. The methodof claim 1, wherein the joint of the at least one parallelogram linkageis a rotatable joint including a torque limiting mechanism foractivating the force limiting device when an input torque generated bythe contact force and transmitted to the torque limiting mechanismexceeds a torque limit equivalent to the activation force.
 3. The methodof claim 1, further comprising: wherein the at least one parallelogramlinkage is configured to be operatively connected to the robot throughone or more rotatable joints; wherein each of the at least oneparallelogram linkage is configured to be operatively connected to thesuspended portion through the one or more rotatable joints; wherein oneof the rotatable joints of each of the at least one parallelogramlinkage includes a torque limiting mechanism for activating the forcelimiting device when the input torque transmitted to the torque limitingmechanism exceeds a torque limit equivalent to the activation force. 4.The method of claim 1, further comprising: wherein the at least oneparallelogram linkage includes a first parallelogram linkage and asecond parallelogram linkage; and wherein the first parallelogramlinkage is perpendicular to the second parallelogram linkage.
 5. Themethod of claim 1, further comprising: wherein the at least oneparallelogram linkage includes at least three parallelogram linkagesarranged in a Delta configuration.
 6. The method of claim 5, furthercomprising: wherein the force limiting device includes a gravitycompensating mechanism.
 7. The method of claim 6, further comprising:wherein the gravity compensating mechanism includes at least one of aspring and a counterbalance.
 8. The method of claim 6, furthercomprising: wherein the gravity compensating mechanism counteracts theweight of at least one of the suspended portion and the payload.
 9. Themethod of claim 1, further comprising: wherein the parallelogram linkageis in a non-compliant state when the contact force is less than theactivation force such that the suspended portion is rigidly fixed in afirst position relative to the robot.
 10. The method of claim 9, whereinactivating the force limiting device further comprises: passively movingthe parallelogram linkage of the activated force limiting device fromthe first position to a second position such that in the second positionthe suspended portion is compliantly attached to the robot by the forcelimiting device.
 11. The method of claim 9, further comprising:preventing movement of the parallelogram linkage from the first positionwhen the contact force is less than the activation force.
 12. The methodof claim 1, further comprising: wherein the robot is configured to bemovably attached to an overhead suspension; and the suspended portion ismovable by the robot toward the object.
 13. The method of claim 12,further comprising: braking the robot relative to the overheadsuspension when the contact force exceeds the activation force.
 14. Amethod for limiting a contact force of a robot with an object, themethod comprising: maintaining a force limiting device of a robot in anon-activated state when a contact force of the robot contacting anobject generates an input torque transmitted through the force limitingdevice which is less than an activation torque; the force limitingdevice comprising: a first attachable portion defining a first end ofthe force limiting device and including an interface configured tofixedly attach the first attachable portion to the robot; a secondattachable portion defining a second end of the force limiting deviceand including an interface connectable to a suspended portion; aparallelogram linkage including two legs; wherein each leg is rotatablyconnected to the first attachable portion and to the second attachableportion; and a torque limiter including an input shaft in communicationwith the parallelogram linkage and activatable from a non-activatedstate to an activated state when the input torque transmitted throughthe parallelogram linkage to the input shaft exceeds the activationtorque; the method further comprising: restraining rotation of the legsrelative to the first attachable portion and the second attachableportion to define a fixed orientation between the first attachableportion, the second attachable portion and the legs when the torquelimiter is in the non-activated state; preventing movement of the secondattachable portion relative to the first attachable portion when theinput torque transmitted to the input shaft is less than the activationtorque; and releasing the legs to passively rotate relative to the firstand second attachable portions to decrease the contact force between themoving object and the contacted object when the torque limiter is in theactivated state.
 15. The method of claim 14, further comprising: whereinthe robot has a moving portion and a suspended portion; wherein thefirst attachable portion is fixedly attached to the moving portion andthe second attachable portion is attached to the suspended portion;wherein the moving portion is configured to move relative to the object;and wherein the orientation between the moving portion and the suspendedportion of the robot is defined by a fixed orientation between the firstattachable portion, the second attachable portion, and the legs when theinput torque defined by the contact force transmitted through one of thesuspended portion and the force limiting device is less than theactivation torque.
 16. The method of claim 15, further comprising:wherein the moving portion is movable by a support system and includes abraking mechanism configured for selectively stopping movement of themoving portion by the support system; the method further comprising:stopping movement of the moving portion by the support system when theinput torque exceeds the activation torque.
 17. The method of claim 15,further comprising: a contact point defined by the location of contactbetween the contacted object and one of the suspended portion and theforce limiting device at a distance from the input shaft; wherein thecontact force is transmitted between the one of the suspended portionand the force limiting device and the contacted object through thecontact point and defines the input torque inputted to the input shaft;the method further comprising: transmitting the input torque through theparallelogram linkage to the input shaft of the torque limiter such thatthe magnitude of the input torque defined by the contact force isindependent of the distance between the input shaft and the contactpoint.